'Asim reported: I asked Anas b. Malik whether Allah's Messenger (ﷺ) had declared Medina as sacred. He said: Yes. (the area) between so and so. He who made any innovation in it, and further said to me: It is something serious to make any innovation in it (and he who does it) there is upon him the curse of Allah, and that of the angels and of all the people, Allah will not accept from him on the Day of Resurrection either obligatory acts or the surpererogatory acts. Ibn Anas said: Or he accommodates an innovator.
Source: sunnah.com
Other translations
Arabic
وَحَدَّثَنَاهُ حَامِدُ بْنُ عُمَرَ، حَدَّثَنَا عَبْدُ الْوَاحِدِ، حَدَّثَنَا عَاصِمٌ، قَالَ قُلْتُ لأَنَسِ بْنِ مَالِكٍ أَحَرَّمَ رَسُولُ اللَّهِ صلى الله عليه وسلم الْمَدِينَةَ قَالَ نَعَمْ مَا بَيْنَ كَذَا إِلَى كَذَا فَمَنْ أَحْدَثَ فِيهَا حَدَثًا - قَالَ - ثُمَّ قَالَ لِي هَذِهِ شَدِيدَةٌ " مَنْ أَحْدَثَ فِيهَا حَدَثًا فَعَلَيْهِ لَعْنَةُ اللَّهِ وَالْمَلاَئِكَةِ وَالنَّاسِ أَجْمَعِينَ لاَ يَقْبَلُ اللَّهُ مِنْهُ يَوْمَ الْقِيَامَةِ صَرْفًا وَلاَ عَدْلاً " . قَالَ فَقَالَ ابْنُ أَنَسٍ أَوْ آوَى مُحْدِثًا .
sunnah.com